1. Field of the Invention
The invention relates to an improvement in a tandem frictional engagement device in which frictional engagement units such as clutches, brakes and the like are axially disposed adjacent to one another, and to an automatic transmission mounted with the tandem frictional engagement device.
2. Description of the Related Art
A great number of vehicular automatic transmissions employing a plurality of planetary gear units and frictional engagement units such as clutches, brakes, and the like have been in use. There is known a tandem frictional engagement device comprising (a) a first frictional engagement unit and (b) a second frictional engagement unit. The first frictional engagement unit couples a first coupling member to a first coupled member through a procedure wherein a friction disc that is relatively non-rotatably spline-fitted to the first coupling member is pressed by a first piston in a first direction and wherein the friction disc and another friction disc on the side of the first coupled member are clamped between the first piston and a first stopper member. The second frictional engagement unit is substantially equal in radial dimension to the first frictional engagement unit, is disposed on the side of the first direction while being coaxial with and adjacent to the first frictional engagement unit, and couples a second coupling member to a second coupled member through a procedure wherein a friction disc that is relatively non-rotatably spline-fitted to the second coupling member is pressed by a second piston in a direction opposite to the first direction and wherein the friction disc and another friction disc on the side of the second coupled member are clamped between the second piston and a second stopper member. One such example is an automatic transmission disclosed in Japanese Patent Application Laid-Open No. 8-61393. In this automatic transmission, first and second coupling members are constructed as a common coupling member, and first and second stopper members are constructed as a common snap ring.
However, if a snap ring 104 is fitted in a straight groove 102 formed in a common coupling member 100, for example, as shown in FIG. 6A, a predetermined gap (looseness) is inevitably created between the straight groove 102 and the snap ring 104. Therefore, it is difficult to manage a piston stroke of adjacent ones of frictional engagement units, and to finely control an engagement torque during a gear shift with high precision. If a snap ring 108 is fitted in a tapered groove 106 having one slanting wall surface as shown in FIG. 6B, there is no gap between the tapered groove 106 and the snap ring 108. However, if a load F is applied leftwards during engagement of a frictional engagement unit on the right side, the snap ring 108 becomes likely to tumble down. This not only makes it difficult to manage a piston stroke of the frictional engagement unit on the right side. Namely, when a frictional engagement unit on the opposite or left side is released, the clearance between friction discs thereof decreases and causes an increase in dragging torque. As a result, a deterioration in transmission efficiency may be caused, or the friction discs may be worn.
Further, if straight grooves 110 and 112 are provided for frictional engagement units on both sides respectively while snap rings 114 and 116 are fitted in the straight grooves 110 and 112 respectively as shown in FIG. 6C, problems such as creation of the aforementioned gap or tumbling are solved. However, a dimension “t” needs to be set between the straight grooves 110 and 112 so as to ensure a predetermined strength. As a result, the overall axial dimension is increased, and the mountability for a vehicle is adversely affected. In such cases where the snap rings are provided separately for the frictional engagement units, even if rattling is caused due to a gap, engagement is always carried out with a constant piston stroke. Thus, it is not absolutely required that the grooves be tapered.
On the other hand, the tandem frictional engagement device of the related art requires that the friction discs of the pair of the frictional engagement units and the like be sequentially mounted to the common coupling member. For example, therefore, in the case where a tandem brake is mounted to a case or the like, the mounting operation is sometimes troublesome and time-consuming.